The present invention is directed in general to semiconductor substrates. The present invention is specifically directed to a semiconductor substrate that includes a neutron conversion layer capable of being utilized for the detection of neutrons. A neutron detection device fabricated in the substrate can be one of many different types including: simple junctions (meta/oxide/metal or p/n junction type), charge-coupled devices (CCDs), and metal-oxide-semiconductor (MOS) integrated circuits.
The development of nuclear weapons gave rise to several urgent applications for highly sensitive neutron detectors. The applications include safeguarding nuclear materials and weapons, treaty verification, anti-proliferation, and the recovery of lost military payloads. More recently, however, the need to guard against nuclear smuggling, the potential of a radiological weapon (so called “dirty” bombs), and terrorist acts, has given rise to an urgent need to perform neutron surveillance at border and port facilities, transportation systems and other places where large amounts of cargo or people passes by or through on a regular basis. Such neutron surveillance must be accomplished without undue restriction or disruption of traffic flow and events.
Conventional neutron detectors have been based on the phenomenon of scintillation, which is a result of electronic transitions that occur in the wake of energetic charged nuclei being released from reactions between incident neutrons and an irradiated atomic nuclei. Scintillation devices include a neutron sensitive material (either a gas or a liquid) that generates charged particles upon receipt of incident neutrons. Typically, gaseous helium-3 contained in glass tubes has been utilized in conventional scintillation devices. The scintillation devices are typically coupled to a photomultiplier tube to generate an analog electrical signal based on the production of the charged particles within the glass tubes. These types of conventional neutron detectors are bulky and not well suited for use in field operations requiring compact and highly sensitive devices. In particular, the helium-3 filled tubes are delicate, require careful handling, and can indicate false positives when abruptly moved or struck.
With the advent of solid state electronics, it was realized silicon-based semiconductor devices could be used as detectors for detecting alpha particles resulting from an (n, alpha) reaction with a neutron converter material. Initial demonstrations of such a concept used free standing converter foils placed near a silicon detector such as a PIN diode. It is more common now to utilize films of converter material placed in contact with or deposited directly upon semiconductor detectors. Lithium metal has been used for this purpose, although the chemical reactivity of the lithium metal tends to lead to shorter detector life. Greater life has been obtained with compounds of lithium such as LiF, a hard crystalline material. Boron metal has also been applied directly to silicon devices. See, “Recent Results From Thin-Film-Coated Semiconductor Neutron Detectors”, D S. McGregor et al., X-Ray and Gamma-Ray Detectors and Applications IV, Proceedings of SPIE, Vol. 4784 (2002), the contents of which are incorporated herein by reference.
The use of diode structures, however, in neutron detectors has its own set of drawbacks and limitations. The internal noise level of an uncooled diode is appreciable, and consequently it is difficult, if not impossible, to measure low background levels of ambient thermal neutrons in the surrounding area. A typical diode also has a thick semiconductor layer in which charges are collected, and is not sensitive enough to detect single neutron events. Charges liberated by gamma rays are also collected in the thick semiconductor layer and these charges contribute to the non-neutron noise signal of the detector.
More recently, it has been proposed that a previously considered disadvantage of semiconductor memory cells be turned into an advantage with respect to neutron detection. Memory cells have traditionally been “hardened” against radiation to prevent errors induced by radiation. In fact, the importance of such memory integrity has been readily appreciated for many years in the field of computers, aviation and space flight. A radiation-induced bit error is known as a soft error if the affected memory cell subsequently responds to write commands. In contrast, the induced bit error is known as a hard error if subsequent attempts to change the state of the memory cell are ineffective. Both hard and soft errors are known as single event upsets (SEUs) or single event errors (SEEs) provided that a single incoming particle induces the error in the memory cell. The error events, which are detrimental when trying to maintain data integrity, can be used in a positive manner to detect radiation events by simply monitoring the states of the memory cells.
Attempts have been made to utilize commercial memory circuits with a neutron converter in order to use the SEU associated with the memory circuits for neutron detection. For example, boron has been used in the semiconductor industry as a dopant and in boron containing glass as a passivation layer that is used to encapsulate a finished semiconductor chip. It has been demonstrated that 10B in the dopant or borophophosilicate glass (BSPG) passivation layer is responsible for sensitizing a circuit to neutron radiation. See, “Experimental Investigation of Thermal Neutron-Induced Single Event Upset in Static Random Access Memories”, Y. Arita et al., Jpn. J. Appl. Phys. 40 (2001) pp L151-153, the contents of which are incorporated herein by reference. Accordingly, proposals have been made to coat boron on a conventional passivated semiconductor memory chip or to mill off the passivation first and then coat the chip with a boron converter layer. U.S. Pat. No. 6,075,261 issued to Houssain et al. and entitled “Neutron Detecting Semiconductor Device”, the contents of which are incorporated herein by reference, discloses one such attempt at utilizing a conventional semiconductor memory structure as a neutron detector, wherein a neutron-reactant material is coated over a conventional flash memory device. These efforts to date, however, have resulted in insensitive detectors primarily because the boron conversion material is not located close enough to the active device layer due to thick overlayers used for interconnection and electrical isolation. Thus, alpha particles and lithium ions generated by the boron conversion material cannot generate a sufficient charge in the active device layer to cause an SEU.
In view of the above, it would be desirable to provide a semiconductor substrate that internally incorporates a neutron conversion layer, which could be utilized for the production of a neutron detection device that does not require the use of fragile tubes or high voltages, is not sensitive to gamma radiations, is not sensitive to thermal noise, but yet is sensitive enough to permit the counting of single neutron events.